Overview

Borehole instabilities during drilling cause substantial problems in all areas of the world. A borehole stability problem is an example of what drillers refer to as a tight hole or stuck pipe incident. There are many possible reasons for being stuck, but in a majority of the field cases reported, the fundamental reason is mechanical collapse of the borehole (see e.g. Bol et al., 1994; Gazianol et al., 1995). Most instabilities of practical importance occur in shale or mudstone; predominantly in the overburden, but sometimes also within the reservoir. Often mechanical hole collapse is combined with a lack of hole cleaning ability. It is a common opinion in the industry that such stability problems amount to typically 5 10% of drilling costs in exploration and production, incorporating loss of time and sometimes also of equipment. These numbers imply a worldwide cost to the petroleum industry of hundreds of million dollars per year.

Borehole stability problems have been encountered for as long as wells have been drilled. Several new challenges have appeared in recent years, making the stability issue more difficult to handle, but also more important to solve. For example, there has been an increasing demand by the industry for more sophisticated well trajectories. Highly deviated, multilateral and horizontal wells are attractive, since a single production platform then can drain a larger area, which can reduce the number of platforms required to produce a given field. Stable drilling is however normally more difficult in deviated than in vertical boreholes. Other...